The present invention relates to reading information from a servo track in a longitudinal tape system, and more particularly, to using a robust metric computation for longitudinal position (LPOS) symbol detection.
Timing-based servo (TBS) systems utilize technology that was developed specifically for linear tape drives. In TBS systems, recorded servo patterns have transitions with two different azimuthal slopes. The head lateral position is derived from the relative timing of pulses generated by a servo reader reading the pattern. The servo patterns are written on five dedicated servo bands that straddle four data bands. A specification for the servo patterns in current tape drives is provided by the linear tape-open (LTO) format. The complete format for LTO drives of generation 1 (LTO-1) was standardized by the European Computer Manufacturers Association (ECMA) in 2001 as ECMA-319. Additional information on LTO technology, in particular on LTO drives of generations 2 to 5 (LTO-2 to LTO-5) is available on the Internet.
TBS patterns allow the encoding of additional LPOS information without affecting the generation of the transversal position error signal (PES) which is used for lateral tape positioning. The LPOS information is obtained by binary pulse-position modulation (PPM) of dibits corresponding to the second and fourth stripes in bursts comprising five stripes. An LPOS word in the LTO format has 36 binary data symbols, or bits, and provides information on a specific absolute longitudinal address. Therefore, an LPOS word appears every 7.2 mm of the tape, i.e., once every 36 servo frames, each of which is 200 μm long and encodes 1 bit of information. The LPOS values of two consecutive LPOS words differ by one. Accordingly, an LTO drive can position the data/servo head assembly down a tape to a given LPOS value with a longitudinal resolution of 7.2 mm by using the LPOS information. The timing-based servo format has not changed over five generations of LTO drives (LTO-1 to LTO-5).
Optimum detection of the servo patterns is achieved by a synchronous servo channel employing an interpolator/correlator, which ensures that optimum filtering of the servo reader signal for extraction of timing information is performed not only at constant tape velocity, but also during acceleration and deceleration. A synchronous servo channel thus ensures the optimum processing of a servo signal for the generation of head lateral position estimates, velocity estimates, and LPOS information. However, the metric for LPOS detection is typically obtained by filtering interpolated samples of the servo signal waveform by a finite-impulse digital filter whose coefficients are obtained by down-sampling an impulse response. Ideally, this is equivalent to computing the metric for LPOS detection as the output of a matched filter at two predefined time instants. This method, however, is optimal only if the down-sampled matched filter response exactly matches the corresponding samples of the servo channel response. If there is a mismatch between the two responses, as for example when tape cartridges exhibit significant deviation of the servo format parameters from the standard values, LPOS detection performance may be severely impaired and tape drive operation compromised.
Furthermore, currently implemented methods do not provide a high margin against noise and other disturbances at high tape velocity, as aliasing effects adversely affect the computation of the metric for LPOS detection based on matched filtering by a finite-impulse response digital filter using fixed filter coefficients from a down-sampled impulse response.